Semiconductor thin film crystallization device and semiconductor thin film crystallization method

ABSTRACT

A first laser beam is emitted from a first laser oscillator in a pulsed manner at a high repetition frequency, and converged onto a substrate by a first intermediate optical system  2  so as to form a slit-like first beam spot. A second laser beam is emitted from a second laser beam oscillator in a pulsed manner to rise precedent to and fall subsequent to the first laser beam, and converged onto the substrate by a second intermediate optical system so as to form a second beam spot similar in configuration to the first beam spot and to contain the first beam spot. Crystallization of a semiconductor thin film on the substrate is carried out while the substrate or the first, second beam spots are moved. Thereby, the whole semiconductor thin film is formed into a crystal surface that has grown in one direction and free from ridges. Thus, the semiconductor thin film has an extremely flat surface, extremely few defects, large crystal grains and high throughput.

CROSS-REFERENCE TO RELATED APPLICATIONS

This nonprovisional application claims priority under 35 U.S.C. §119(a)on Patent Application No. 2004-217089 filed in Japan on 26 Jul. 2004,the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor thin filmcrystallization device and a semiconductor thin film crystallizationmethod, for crystallizing semiconductor thin film by using laser.

In recent years, thin film transistors (hereinafter, referred to asTFTs) using polysilicon film have been drawing attention among TFTsunder development. In particular, LCDs (Liquid Crystal Displays) or EL(Electro-Luminescence) Displays employ TFTs in which polysilicon film isused as elements for switching pixels or as elements that form part ofthe driver circuit for controlling the pixels.

Generally, as the method for obtaining the polysilicon film, amorphoussilicon film is crystallized to provide polysilicon film. Recently,attention has been given particularly to a method of crystallizingamorphous silicon film by using laser light. In this case,crystallization by laser makes it possible to achieve thecrystallization by heating only the semiconductor film such as theamorphous silicon film. Therefore, this method is an effective methodfor forming a crystallized semiconductor film on a substrate of low heatresistance such as glass substrates or plastic substrates. Thiscrystallization method for semiconductor film by using laser light isdisclosed in detail in JP 2001-44120 A (hereinafter, referred to aspatent document 1), JP H11-307450 (patent document 2) and JP 2000-505241A (patent document 3).

The patent document 1 describes a laser heat treatment device thatimproves the crystallinity by using a plurality of light sources. Thedocument further describes, in particular, the use of an ultraviolet rayas a main light source and a pulsed beam emitted from a solid statelaser as a subordinate light source.

The patent document 2 discloses a thin film reformer for performingcrystallization with radiation of two types of laser light as in thelaser heat treatment device of the patent document 1. The document 2further describes, in particular, that laser light which does not showlarge absorption for semiconductor film but shows large absorption forthe substrate, such as carbonic acid gas laser, is used to improve thecrystallinity of the substrate, which makes it possible to improve thecharacteristics of transistors or the like fabricated on the substrateof improved crystallinity.

The patent document 3 discloses a crystallization process of asemiconductor region on the substrate wherein a linear or slit-shapedbeam is applied to a semiconductor region on the substrate in thelateral direction so as to make such crystal growth that crystals of thesemiconductor film are grown largely in the lateral direction. However,in the case of the crystallization process for the semiconductor regionon the substrate disclosed in the patent document 3, the distance oflateral growth by one time irradiation is about 1 micron to 2 microns.Therefore, the above-described crystal growth needs to be repeated asrequired in order to crystallize a large-area semiconductor film.

Various laser devices are available for laser devices to be used for themethods described above. First of all, in terms of the form ofoscillation, laser devices are roughly divided into pulsed laser devicesthat perform pulsed oscillation and continuous wave laser devices thatperform continuous oscillation. Although both devices are used forcrystallization of the semiconductor film, yet the pulsed laser devicesare widely used for crystallization of semiconductor film because of anadvantage capable of instantaneously giving large power.

Currently, pulsed oscillation type excimer laser devices are availableas laser devices which are commonly used for crystallization of thesemiconductor film. In the devices, the repetition frequency of pulsedoscillation is about 1 Hz to 300 Hz. The excimer laser device is largein output power, and the oscillation light is high in absorptioncoefficient for silicon film because of the oscillation light beingultraviolet rays. Moreover, the oscillation light of the excimer laserdevice is capable of instantaneously heating by virtue of their shortpulse width. Thus, the excimer laser device has an advantage that makingthe semiconductor film fused does not involve so much increase in thesubstrate temperature.

However, excimer laser devices need such gas as krF (wavelength: 248 nm)or XeCl (wavelength: 308 nm) for oscillation, and gas supply units forthese gases are expensive. Further, since replacement of gas,replacement of oscillating tubes, replacement of optical windows and thelike are regularly necessitated. This disadvantageously leads to highmaintenance cost.

Further, other laser devices with the medium given by argon gas orcarbonic acid gas have also been used as gas laser devices. Inparticular, carbonic acid gas laser devices are high in efficiency,allowing high output power to be obtained with relatively small-sizedequipment.

Besides, it is also possible to use laser light derived from anoscillation source given by a solid state laser device (a laser devicethat outputs laser light with a crystal rod used as its resonant cavity). Such solid state laser devices are given by commonly known ones, beingexemplified by YAG lasers (which normally mean Nd:YAG lasers), Nd:YVO₄lasers, Nd:YAlO₃ lasers, ruby lasers, Ti:sapphire lasers, glass lasersand the like. Since YAG lasers have a fundamental wave (first higherharmonic) whose wavelength is as long as 1064 nm, the second harmonic(wavelength: 532 nm), the third harmonic (wavelength: 355 nm) or thefourth harmonic (wavelength: 266 nm) is used in some cases. It is notedthat the fundamental wave can be modified to the second harmonic, thethird harmonic or the fourth harmonic by a wavelength modulatorincluding nonlinear elements. The formation of those harmonics isperformed according to known techniques.

Also, in some cases, the Q-switching method (Q-modulation switchingmethod) is used. The Q-switching method is often used for the YAGlasers. This is a method that the Q value is abruptly increased from asufficiently low Q value state of the laser resonator to produce a sharppulsed laser of quite high an energy value. In this method, therepetition frequency of the pulsed oscillation is 100 to several tenskHz. These are known techniques.

Although various semiconductor film crystallization methods using laserdevices have been proposed as described above, there have been alsoproposed methods for crystallizing a semiconductor film in combinationwith the plural kinds of laser devices as well as a single laser device.

Further, in the prior art, there have been provided many proposals forthe method of crystallinity improvement and the method of throughput(processing speed per unit time) improvement in the process ofperforming the crystallization of semiconductor film by using laserlight.

However, the prior arts described in the aforementioned patent documents1 to 3 have the follow disadvantages.

Both the patent documents 1 and 2 describe that the crystallinity of thesemiconductor film can be improved with the use of two types of laserlight. In particular, the patent document 1 discloses in detail therelations among the irradiation intensity of laser light, the mobilityof transistors and the size of crystal grains. However, as an example,the mobility is 100 cm²/Vs to 150 cm²/Vs at most, the value being verylow as compared with single crystal silicon or the like.

This is because only using plural types of laser light does not allowacceleration of the crystal growth so much, and therefore, to achieve adrastic increase of the size of crystal grains, as stated in the patentdocument 1. As a consequence, it is impossible to improve thecharacteristics of transistors by improved crystallinity of thesemiconductor film.

In the patent document 3, laterally elongated crystals are made,thereby, transistors are formed with the channel direction coincidentwith the growth direction of the crystals, which makes it possible tofabricate transistors having a mobility of 300 cm²/Vs to 400 cm²/Vs orover. However, the length of crystals grown by one-time irradiation is 1micron to 2 microns as described above, and therefore, there is a needfor stringing the crystals one after another, which leaves a great issueunsolved in terms of throughput.

Furthermore, in the prior arts disclosed in the patent documents 1 to 3,there occurs a protrusion, so called “ridge”, at each grain boundaryportion of the formed crystals. The ridge is caused by collisions ofcrystals that have grown in different directions. The ridge has a heightcomparable to film thickness of the semiconductor film to becrystallized. Then, if the channel portion of a transistor for exampleis formed at a portion where the ridge is generated, there occurs aphenomenon that electric fields concentrate to the ridge (protrusion)portion to incur a breakdown, which gives rise to an issue ofdeterioration in reliability of the transistors. Besides, the ridgeportion is thick in film thickness and has defects concentratedthereabout, transistors whose channel is formed at the ridge portion aregenerally poor in characteristics and not for practicable use.

In addition, the patent document 3 discloses a method where a transistoris formed in no ridge region which is formed between lateral crystalsafter stringing the lateral crystals one after another. Though it ispossible to form a transistor in no ridge region, deterioration ofthroughput is inevitable in ensuring large areas free from ridges.Besides this, transistors need to be placed so as not to overlap theridges. In this case, the transistor is placed after the completion ofcrystallization and forming a pattern. Therefore, it is necessary topredict the ridge positions preparatorily with an extremely highaccuracy and place the ridges so that interference with the ridges doesnot occur in the later process of forming the transistors. The placementaccuracy of the ridges in such a case needs to be, generally, equivalentto that of the placement of the transistors. For this purpose, equipmentfor performing the crystallization process in semiconductor regions onthe substrate generally requires a level of accuracy equivalent to thatof exposers for pattern formation, thus the price of the equipment beingvery expensive.

In the method of forming the lateral crystal growth in the semiconductorregions on the substrate disclosed in the patent document 3, fabricationof flat crystals free from ridges is hard to achieve under practicalconditions although achievable under certain limited conditions.

In order to manufacture high-performance thin film transistors, it isnecessary to obtain crystals having flat surfaces, less defects andlarge crystal grains at high throughput. However, it is hard to make thecrystals that satisfy these characteristics at the same time in theprior arts disclosed in the patent documents 1 to 3. That is, from thepatent documents 1 to 3, it is impossible to obtain a practicalcrystallization method which satisfies both the obtainment of crystalshaving high-performance crystallinity: less defects and less grainboundaries and flat surfaces, and the capability of high throughput atthe same time even if any types of laser light either alone or incombination is used. In other words, the prior arts disclosed in thepatent documents 1 to 3 make it possible only to obtain crystals havinga capability of fabricating thin film transistors for liquid crystalpanel used at a level of 100 cm²/Vs to 200 cm²/Vs in mobility.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a semiconductor thinfilm crystallization device and a semiconductor thin filmcrystallization method both of which are capable of obtaining channelsurface having extremely flat surfaces and extremely few defects andlarge crystal grains and also capable of obtaining high throughput.

In order to achieve the above-mentioned object, the present inventionprovides a semiconductor thin film crystallization device comprising:

a first laser irradiation unit for emitting a first laser beam andforming a striped first beam spot having a specified width and aspecified length on a semiconductor thin film on a substrate;

a second laser irradiation unit for emitting a second laser beam andforming a second beam spot having a shape containing the first beam spoton the semiconductor thin film; and

a relative moving unit for moving the substrate in a widthwise directionof the first beam spot relatively with respect to the two beam spots,wherein

the second laser irradiation unit emits the second laser beam at leastwhile the first laser beam is being emitted, and maintains intensity ofthe second laser beam constant at a level lower than a maximum intensityof the first laser beam and during a period from an emission start timeto an emission end time, and

a light intensity distribution by the second laser irradiation unitwithin the second beam spot is constant at least in a portionoverlapping with the first beam spot.

In this case, the second laser beam emitted from the second laserirradiation unit can be worked subserviently to the first laser beamemitted from the first laser irradiation unit. Therefore, temperatureincreases of the whole substrate can be prevented, so that deformationof the substrate due to thermal expansion can be prevented.

Further, the semiconductor thin film on the substrate is fused from astart of one time irradiation with the first laser beam and the secondlaser beam, and cooled after the irradiation so as to be solidified andcrystallized. The fusion of the semiconductor thin film in this caseoccurs generally within the first beam spot. The crystallization andsolidification make progress in the widthwise direction of the firstbeam spot from both edge portions toward the central portion.Accordingly, since crystals grow from the two edge portions of the firstbeam spot to the generally central portion, it is possible to obtaincrystals having an average growth length more than several times,specifically about ten times, larger than the conventional averagegrowth length described in the patent document 3. Thus, according to thepresent invention, tenfold throughput can be obtained as compared withthe conventional crystallization device, so that the invention leads toan extremely high productivity.

Furthermore, by moving the substrate in the widthwise direction of thefirst beam spot relative to the first and second beam spots, it becomespossible to obtain continuously, laterally grown crystals while takingover the crystals that have grown by one time irradiation with the firstand second laser beams. In this case, it is possible to achieve thecrystallization in shorter time all over the semiconductor thin film byspeeding up the relative moving speed of the substrate. This is becausecrystals of about ten times the conventional growth length can beobtained by one time irradiation with the first and second laser beamsas described above. Further, according to the present invention, thecontinuous crystallization can be implemented from one end portion tothe other end portion of the semiconductor thin film, and therefore, itis possible to grow crystals free from the presence of ridgestherebetween so that their surfaces are extremely flat.

That is, according to this invention, crystals can be obtained whichhave extremely few defects and which are good in quality and close tothe single crystal. Also, as a result of the obtainment of crystals ofless defects, there can be obtained an effect that crystal defects haveless influence no matter how the channel of the transistors is oriented,thus eliminating constraints on the orientation of the transistorplacement.

In one embodiment of the present invention, the specified width in thestriped first beam spot is not less than 5 micron and not more than 50microns.

As described above, the semiconductor thin film crystallization deviceof this invention is capable of obtaining crystals whose growth lengthis about ten times the conventional average growth length, morespecifically, crystals whose growth length is 2.5 microns to 25 micronsfor one time irradiation. Thus, according to this embodiment, there canbe obtained crystals of large crystal grains which have grown from bothedge portions to generally central portions in the widthwise directionof the first beam spot by one time irradiation with the first laserbeam.

In one embodiment of the present invention, the second laser beamemitted by the second laser irradiation unit is a carbonic acid gaslaser beam.

According to this embodiment, it is possible to use a laser beam ofpulsed oscillation as the second laser beam.

In one embodiment of the present invention, the second laser irradiationunit emits the second laser beam in a pulsed manner.

According to this embodiment, the second laser beam is a laser beam ofpulsed oscillation. Setting the second laser beam to a shorterirradiation time makes it possible to heat the substrate in a statecloser to heat-insulation. As a result of this, the heat escape of thesecond laser beam due to thermal diffusion can be reduced before theirradiation with the first laser beam. Thus, the region to be heated islimited to the surface of the substrate, which makes it achievable toheat only the substrate surface with a less amount of heat.

In one embodiment of the present invention, emission frequency of thefirst laser beam by the first laser irradiation unit is not less than 1kHz and not more than 100 kHz.

According to this embodiment, since the emission frequency of the firstlaser beam is as high as 1 kHz to 100 kHz, it is possible to enhance themoving speed of the substrate, so that the productivity is improved. Themoving speed of the substrate is determined by “moving distance for onetime irradiation” multiplied by “repetition frequency of oscillation offirst and second laser beams.”

In one embodiment of the present invention, the relative moving unit isa substrate driver section on which the substrate is placed and which isenabled to move the substrate from one end portion to other end portionof the substrate in a widthwise direction of the beam spot and to rotatethe substrate by 90°.

According to this embodiment, after one side portion of thesemiconductor thin film on the substrate is crystallized in a stripshape along the one side (first strip crystallization), it is possibleto rotate the substrate by 90° and to crystallize a plurality of stripregions extending from the one side portion of the semiconductor thinfilm toward the other side portion (second strip crystallization), withthe start position given by a position within the crystallized stripregion. Thus, the start position for performing the second stripcrystallization is located within the crystallized strip region, andtherefore, a crystal in the second strip crystallization starts to growwith the start point given not by crystal nuclei generated randomly fromend portions but by a crystal elongated in the longitudinal direction ofthe crystallized strip region. As a consequence, the crystal that growsby the second strip crystallization becomes a single crystal of anextremely large width or almost such a crystal.

The present invention also provides a semiconductor thin filmcrystallization method comprising the steps of:

irradiating a semiconductor thin film on a substrate with a first laserbeam to form a striped first beam spot having a specified width and aspecified length on the semiconductor thin film;

irradiating the semiconductor thin film with a second laser beam to forma second beam spot having a shape containing the first beam spot on thesemiconductor thin film; and

moving the substrate relative to the two beam spots in a widthwisedirection of the first beam spot to crystallize the semiconductor thinfilm on the substrate by the first and second laser beams, wherein

the irradiation with the second laser beam is performed at least whilethe irradiation with the first laser beam is being performed,

intensity of the second laser beam is maintained constant during aperiod from an emission start time to an emission end time, and

light intensity of the second laser beam within the second beam spot ismaintained constant at least over portions overlapping with the firstbeam spot.

According to the embodiment, temperature increases of the wholesubstrate can be prevented by working subserviently the second laserbeam to the first laser beam, which resultantly prevents deformation ofthe substrate due to thermal expansion. Further, the average growthlength of crystal can be made about ten times the conventional averagegrowth length described in the patent document 3. Accordingly, tenfoldthroughput can be obtained as compared with the conventionalcrystallization device. Besides, by moving the substrate relatively inthe widthwise direction of the first beam spot, continuously laterallygrown crystals can be obtained. In this case, it becomes possible toachieve the crystallization in shorter time all over the semiconductorthin film by speeding up the relative moving speed of the substrate.Further, the continuous crystallization can be implemented from one endportion to the other end portion of the semiconductor thin film, andthere can be grown crystals which are free from the presence of ridgestherebetween so that their surfaces are extremely flat.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention, and wherein:

FIG. 1 is a view showing a configuration of a semiconductor thin filmcrystallization device according to the present invention;

FIG. 2 is a view showing a shape of a beam spot formed by thesemiconductor thin film crystallization device shown in FIG. 1;

FIGS. 3A and 3B are views showing relationship between irradiation timeand intensity of a first laser beam derived from a first laseroscillator and a second laser beam derived from a second laseroscillator in FIG. 1;

FIG. 4 is a view showing a crystallization state in a case where asemiconductor thin film is irradiated with the beam spot shown in FIG.2;

FIG. 5 is a view showing a relationship between moving distance of thebeam spot and the crystallization state;

FIG. 6 is a view showing crystals obtained by the first crystallizationmethod in which the substrate or the beam spot is moved;

FIG. 7 is an explanatory view of the second crystallization method;

FIGS. 8A and 8B are views showing a specific configuration of a firstintermediate optical system in FIG. 1;

FIGS. 9A and 9B are views showing a specific configuration of the firstintermediate optical system other than that of FIGS. 8A and 8B;

FIGS. 10A and 10B are views showing a specific configuration of thefirst intermediate optical system other than those of FIGS. 8A and 8Band FIGS. 9A and 9B;

FIG. 11 is a view showing a specific configuration of a secondintermediate optical system in FIG. 1;

FIG. 12 is a view showing a shape of a second beam spot derived from thesecond intermediate optical system shown in FIG. 11;

FIGS. 13A to 13C are views showing an action of an intensityuniformizing element in FIG. 11 on light beams;

FIGS. 14A to 14C are views showing an action of an intensityuniformizing element on light beams other than in FIGS. 13A to 13C;

FIG. 15 is a view showing a shape of a second beam spot by the intensityuniformizing element (holographic element) shown in FIGS. 14A to 14C;

DETAILED DESCRIPTION OF THE INVENTION

Hereinbelow, the present invention is described in detail with referenceto the accompanying drawings. FIG. 1 is a view showing a configurationof a semiconductor thin film crystallization device of this embodiment.The semiconductor thin film crystallization device is made up a firstlaser oscillator 1, a first intermediate optical system 2, a secondlaser oscillator 3, a second intermediate optical system 4 and asubstrate drive unit 5. Then, a substrate 6 placed on the substratedriver unit 5 is irradiated with laser beams derived from the firstlaser oscillator 1 and the second laser oscillator 3.

The first laser oscillator 1 and the first intermediate optical system 2constitute an example of the first laser irradiation unit. The secondlaser oscillator and the second intermediate optical system 4 constitutean example of the second laser irradiation unit. The substrate driverunit 5 constitutes an example of a relative moving unit.

Although not shown in FIG. 1, the semiconductor thin filmcrystallization device may include a variable attenuator for correctingbeam intensity, a shutter for interrupting the beam, various types ofdetectors for measuring laser intensity, an adjustment unit forsubstrate temperature, chamber units for controlling the gas atmospherearound the substrate, and the like.

Further, a semiconductor thin film (not shown) is formed on thesubstrate 6. The semiconductor thin film is crystallized bysolidification after fusion. It is noted that silicon thin film oramorphous silicon thin film or other semiconductor materials may be usedas the semiconductor thin film.

In this embodiment, the semiconductor thin film on the substrate 6 iscrystallized by using the semiconductor thin film crystallization devicehaving the above-described constitution. The method for thiscrystallization includes the supplying of energy necessary for thefusing of the semiconductor thin film primarily by irradiation with alaser beam derived from the first laser oscillator 1, and the giving ofa preliminary effect by irradiation with a laser beam derived from thesecond laser oscillator 3. Then, by a composite effect of theirradiation with the two laser beams, the method simultaneously achievesthe crystal growth ten or more larger than the conventional counterpart,crystal quality of less defects and ridge-free flat crystal surfaces, aswell as high throughput.

First, a first crystallization method according to this embodiment isexplained.

The first laser oscillator 1 may be a laser oscillator of eithercontinuous oscillation or pulsed oscillation. However, it is preferableto use a laser oscillator that performs pulsed oscillation to emit apulsed energy beam. Also, the first laser oscillator 1 is notparticularly limited in its light source type if its emission wavelengthcan be set to within a range of 550 nm to 200 nm so that a certaindegree or higher of absorption for silicon is given to make the fusingachievable. For example, the light source is desirably implemented byvarious types of solid state laser devices, which are typified byexcimer laser devices that oscillate ultraviolet rays or YAG laserdevices that converts infrared oscillated light into a doubled harmonicwave, or solid state ultraviolet laser devices that have wavelengths ofthe ultraviolet region such as the harmonics.

Further, as the first laser beam emitted from the first laser oscillator1, it is the most desirable to use a beam having the pulse width of 10ns to 200 ns and the oscillation frequency of 1 kHz to 100 kHz with useof a solid state laser device which performs pulsed oscillation at thewavelength region (550 nm to 200 nm). For such a laser device, it ispossible to use second harmonics of the YAG lasers (which normally meanNd:YAG lasers), Nd:YVO₄ lasers, Nd:YAlO₃ lasers, ruby lasers,Ti:sapphire lasers, glass lasers and the like.

The second laser oscillator 3 is a laser oscillator which gives apreliminary effect for the fusing of the semiconductor thin film asdescribed above, and a laser oscillator of either continuous oscillationor pulsed oscillation is usable therefor. The second laser oscillator 3is preferably the one providing a pulsed energy beam. However, thesecond laser oscillator 3 does not necessarily need to be a laseroscillator that performs pulsed oscillation. A laser beam of continuousoscillation may be modulated by a proper method.

Also, the emission wavelength of the second laser oscillator may be theone that can be absorbed into silicon in a certain degree or higher soas to fuse silicon. However, other wavelengths are usable as well. Forinstance, as the second laser device, it is possible to use varioustypes of solid state laser devices typified by excimer lasers or YAGlasers, or gas laser devices of carbonic acid gas lasers or argon ionlasers or the like.

The most desirable wavelength as the second laser oscillator is awavelength which yields the more absorption at the substrate 6. Morespecifically, the second laser beam is given by using a carbonic acidgas laser device having an emission wavelength of about 10 microns andby a pulse width of 1 μs to 100 μs and a pulse repetition frequencyequal to that of the first laser beam (i.e., 1 kHz to 100 kHz). Such alaser beam can be obtained normally by modulating a CW carbonic acid gaslaser.

FIG. 2 shows a preferable spot shape at a laser beam irradiationsurface, which is formed by the semiconductor thin film crystallizationdevice of this embodiment. Referring to FIG. 1, a first laser beamemitted from the first laser oscillator 1 is converged onto thesemiconductor thin film on the substrate 6 by the first intermediateoptical system 2 to form a slit-shaped first beam spot 11 having aspecified length and a specified width. The width in this case is 5microns to 50 microns. The length is preferably so as to yield enoughintensity to fuse the semiconductor thin film on the substrate 6 withsuch a width of 5 microns to 50 microns and to be as long as the powerof the first laser oscillator 1 permits, from the viewpoint of reductionin the time required for crystallization process.

Generally, amorphous silicon is used as the semiconductor thin film onthe substrate 6, its thickness being 50 nm. In this case, when a solidstate laser device having a wavelength of 532 nm and a pulse width of 10ns to 100 ns is used as the laser device for the first laser oscillator1, fusion occurs with energy of 200 mJ/cm² to 600 mJ/cm² uponirradiation with only the first laser beam. Therefore, designing theshape of the first beam spot 11 to yield an energy density roughly ofthe above level makes it possible to properly fulfill thecrystallization.

A light intensity distribution at the beam spot (first beam spot) 11 onthe surface of the semiconductor thin film, which is formed by the firstlaser beam from the first laser oscillator 1, has a rise (fall) of theintensity preferably as abruptly as possible at edge portions in theshort side (width) direction of the first beam spot 11. Desirably, adistance over which the intensity changes from 10% to 90% is about 2microns or less. On the other hand, in the longer side (length)direction of the first beam spot 11, it is desired that the lightintensity is constant, its variations desirably being ±5% or less. Theedge portions in the longer side direction of the first beam spot 11desirably have the same rises (falls) as those of the edge portions inthe shorter side direction.

The shape of a beam spot (second beam spot) 12, which is formed on thesurface of the semiconductor thin film by the second laser beam derivedfrom the second laser oscillator 3, is set to a shape generally similarto the spot shape of the first beam spot 11. Preferably, the second beamspot 12 has such a size and a shape that at least the first beam spot 11is contained therein geometrically. Also, desirably, the light intensitydistribution by the second laser beam in the second beam spot 12 isconstant at its portion overlapping with the first beam spot (includingrising portion and falling portion).

It is noted here that the terms “light intensity (distribution)constant” as mentioned above mean that the light intensity distributionis constant from a design's point of view, and so regarded as constanteven if the intensity varies to some extent depending on manufacturevariations.

FIGS. 3A and 3B show relationships in irradiation time and intensitybetween the first laser beam derived from the first laser oscillator 1and the second laser beam derived from the second laser oscillator 3. Inthe case of this embodiment, the intensity versus irradiation timerelationship between the first laser beam and the second laser beam canbe given mainly in two types.

As to how to give the intensity in relation to first irradiation time,the first laser beam is emitted as pulsed irradiation while the secondlaser beam is continuously emitted as continuous irradiation, as shownin FIG. 3A (first method). It is noted here that the intensity of thesecond laser beam is a specified intensity lower than the highestintensity of the first laser beam. The pulse interval of the first laserbeam in this method depends on the repetition frequency of oscillationof the first laser oscillator 1. As the first laser device, a solidstate laser device of pulse oscillation is used, which is YAG lasers(which normally mean Nd:YAG lasers), Nd:YVO₄ lasers, Nd:YAlO₃ lasers,ruby lasers, Ti:sapphire lasers, glass lasers, etc. In the case of usingits second harmonics, the repetition frequency of oscillation can be setto 1 kHz to 100 kHz.

As to how to give the intensity in relation to second irradiation time,the second laser beam is emitted as pulsed irradiation while the firstlaser beam is emitted in a pulsed manner as well as shown in FIG. 3B(second method). It is noted here that the intensity of the second laserbeam is a specified intensity lower than the highest intensity of thefirst laser beam. In this case, a laser device equivalent to that of thefirst method can be used for the first laser beam, where the secondlaser beam is emitted in synchronization with the first laser beam. Forthis purpose, the pulse width of the second laser beam is set largerthan the pulse width of the first laser beam. Then, the timing ofirradiation start with the second laser beam is made at least precedentto the timing of irradiation start with the first laser beam. Also, thetiming of irradiation end with the second laser beam is preferably setlater than the timing of irradiation end with the first laser beam.

In the second method, the irradiation time (pulse width) with the secondlaser beam is preferably set as short as possible. With this setting,the irradiation energy density of the second laser beam per unit areaand per unit time needs to be set higher according to the reduction inpulse width. On the other hand, the setting of a shorter irradiationtime makes it possible to reduce the input amount of heat into thesubstrate 6, the input amount of heat being expressed as “pulse width”multiplied by “irradiation energy density.” The reason for this is thatthe setting of a shorter irradiation time (pulse width) allows heatingperformed by the second laser beam to be heating almost in aheat-insulated state. As a result, it is possible to reduce the heatwhich escapes due to thermal diffusion before the irradiation of thefirst laser beam, and therefore, to limit the heated region to thesurface of the substrate 6, thus making it achievable to heat only thesurface with a small amount of heat. Thus, the second method has aneffect of preventing increase of temperature in the whole substrate 6,so that deformation of the substrate 6 due to thermal expansion can beprevented.

Next, the explanation is given as to the crystal state resulting fromthe crystallization of the semiconductor thin film on the substrate 6performed under the above-described settings, and the drive method forthe substrate in such case. FIG. 4 shows a state of crystals in a casewhere the semiconductor thin film is irradiated with the first laserbeam and the second laser beam. As shown in FIG. 4, together with theirradiation of the first laser beam and the second laser beam, thesemiconductor thin film on the substrate 6 fuses and, starting to becooled after the termination of the irradiation, yielding solidificationand crystallization. The fusion of the semiconductor thin film in thiscase occurs generally within the region of the first beam spot 11.Crystal solidification, which starts from the crystal nuclei and whichis generated randomly from end portions, progresses from both-side edgeportions of the region of the first beam spot 11. As a result of this,as shown in FIG. 4, there can be obtained a plurality of crystals 15which have laterally grown in the widthwise direction of the first beamspot 11 and toward the center portions. In this embodiment, while thislateral crystal growth is kept occurring, irradiation of the secondlaser beam is applied compositely. This causes extremely large crystalgrowth. It is noted that the growth length of the crystals 15, althoughvarying depending on conditions, is 2.5 microns to 25 microns for eachone time of irradiation, which is approximately 2.5 to 25 times, aboutten times on the average, the growth length described in the patentdocument 3 (1 micron to 2 microns).

In addition, that the growth length 16 for each one time of irradiationis ten times on the average that of the patent document 3 means thattenfold throughput can be obtained at all times as compared with thepatent document 3. Thus, it can be said that an extremely highproductivity can be obtained according to this embodiment.

More specifically, a solid state laser device having a wavelength of 532nm, a pulse width of 30 ns and an irradiation intensity of 300 mJ/cm² isused as the first laser device. A carbonic acid gas laser device havinga wavelength of 10.6 μm, a pulse width of 100 μs and an irradiationintensity of 40 W/mm² is used as the second laser device. Then, in acase where the width of the first and second beam spots is about 20microns, a crystal growth length of 10 microns for each one time ofirradiation can be obtained.

In this embodiment, the width of the first beam spot 11 is set to 5microns to 50 microns as described above. Accordingly, on the basis thatthe crystal growth length 16 of 2.5 microns to 25 microns for each onetime of irradiation can be obtained, it is implementable to obtaincrystals 15 that have generally grown up to the roughly center portionof the irradiation region of the first beam spot 11.

Whereas the crystallization yielded by one time of irradiation with thelaser beam is as described above, such crystallization is performedwhile the substrate 6 is being moved (or the first beam spot 11 and thesecond beam spot 12 are being moved in equal direction and at equalspeed) in this embodiment.

FIG. 5 shows a relationship between the shape of the beam spots 11, 12and the moving distance 17. The move of the substrate 6 (or beam spots11, 12) may be either intermittent or continuous. For instance, as apreferred example, when the pulse width of the first laser beam is setto 10 ns to 200 ns, the distance to which the substrate 6 moves isextremely small with practical speed. Therefore, the move of thesubstrate 6 can be done continuously without being stopped for each timeof irradiation, which makes it possible to adopt a simple method thatthe substrate 6 is continuously fed. However, in either case ofintermittent or continuous feed, the distance 17 to which the substrate6 is moved for each one time of irradiation is set to not less than ½time and not more than 1 time the growth length of crystals by roughlyone time of irradiation. Otherwise, the distance is set to not less than¼ time and not more than ½ time the width of the first beam spot 11.

The moving speed of the substrate 6 in this case is determined by“moving distance for one time irradiation” multiplied by “repetitionfrequency of oscillation of first, second laser oscillators 1, 3.”Accordingly, typically, the moving speed is 10 microns×10 kHz=100 mm/s,which means that the whole substrate 6 can be crystallized at apractical high speed. For example, even in the case of a several hundredmillimeter square glass substrate, the substrate 6 can be irradiatedfrom its one end to the other end in several seconds.

As the result, as shown in FIG. 6, a plurality of crystals 18 extendingin one way and free from ridges can be obtained on the substrate 6 byusing such a crystallization method as described above, in whichirradiation with the single linear-shaped first beam spot 11 is appliedat a relatively high repetition frequency, in which large crystal growthis yielded by the second beam spot 12, and in which the crystallizationis carried out while the substrate 6 or the beam spots 11, 12 are moved.In this case, since there are no ridges in the region to which thesubstrate 6 (beam spots 11, 12) is moved, flat crystals 18 are obtained.Thus, when the substrate 6 (beam spots 11, 12) is moved from one end tothe other end of the substrate 6, a thoroughly ridge-free, flatcrystallized region is obtained from one end to the other end of thesubstrate 6. That is, it is substantially possible to obtain the entiresubstrate 6 having unidirectionally grown crystals and a ridge-freesurface.

As described above, the first crystallization method of this embodimentmakes it possible to yield ten or more times the conventional crystalgrowth by applying a composite irradiation with the second beam spot 12(most desirably, a beam spot by a carbonic acid gas laser beam) inaddition to the linear irradiation having a width of 5 micron to 50microns by the first beam spot 11. Accordingly, by iterating theirradiation with the beam spots 11, 12 a plurality of times andsimultaneously moving the substrate 6, it becomes implementable tofabricate a high-quality, ridge-free crystal thin film on the wholesubstrate 6 at practical speed.

Consequently, according to this embodiment, taking over the growncrystals makes it possible to obtain continuously, laterally growncrystals. Also, doing the crystallization at high speed as describedabove makes it possible to implement the crystallization over the wholesemiconductor thin film on the substrate 6. Further, since thecontinuous crystallization can be implemented from one end portion tothe other end portion of the semiconductor thin film on the substrate 6,it is implementable to obtain crystals which are free from the presenceof ridges therebetween so that their surfaces are extremely flat.

Accordingly, transistors of quite high characteristics are obtained inthe case where the transistors are formed by using the substratecrystallized according to this embodiment. For instance, after stackinga SiO₂ ground layer and amorphous silicon (50 nm) on a glass substrate,the crystallization of the amorphous silicon is performed according tothis embodiment. In the case where the channel direction of thetransistors are formed in line with the growth direction of thiscrystallization, a mobility of 300 cm²/Vs to 400 cm²/Vs can stably beobtained. This is equivalent to two to three times the mobilitydisclosed in the patent document 1, which is a dramatic improvement.Furthermore, as to the irregularities of the surface, there are noridges and the average roughness is not more than 5 nm, so that theaverage roughness can be made not more than 1/10 of normal averageroughness.

Next, the second crystallization method according to this embodiment isexplained. Also for this second crystallization method, thesemiconductor thin film crystallization device shown in FIG. 1 is used.

As shown in FIG. 7, first of all, irradiation with the first and secondlaser beams is performed at least one time to a first strip region 21,which ranges from one end portion to the other end portion of thesubstrate 6, along one edge portion close to one side out of the foursides of the semiconductor thin film 20 on the substrate 6. Thereby,first strip crystallization of the semiconductor thin film is performed.In this process, the width of the first strip region 21 is generallyequal to the length of the first beam spot (see FIG. 2).

Next, the substrate 6 rotated 90°, and thereafter, irradiation with thefirst and second laser beams is performed to a second strip region 23,which ranges from one end portion to the other end portion of thesemiconductor thin film 20, the one end portion being within the firststrip region 21 serving as a start point (more precisely, the startpoint is a position 22 within the first strip region 21). Thereby,second strip crystallization of the semiconductor thin film isperformed. In this case also, the width of the second strip region 23 isgenerally equal to the length of the first beam spot. After this on, thesecond strip crystallization on the second strip region 23 is iterated aplurality of times, by which crystallization all over the substrate 6 isachieved.

According to this second crystallization method, the start point 22 forthe second strip crystallization is positioned within the first stripregion 21. Since crystals elongated in the longitudinal direction of thefirst strip region 21 have already grown at the portion of the startpoint 22, crystal growth of the second strip region 23 starts with thecrystals in the first strip region 21 as the seeds. Accordingly, thecrystals that grow in the second strip region 23 are not those whichgrow with the start point given by crystal nuclei generated randomlyfrom end portions, but those which start to grow with the start pointgiven by the crystals elongated in the longitudinal direction of thefirst strip region 21. For this reason, the crystal that grows in thesecond strip region 23 is not the plurality of elongated crystals thathave grown as in the case of the first strip region 21, but a crystal ofan extremely large width or almost such a crystal.

Thus, the crystals formed in the semiconductor thin film by the secondstrip crystallization are those of good quality having extremely fewdefects and close to the single crystal. Also, as a result of reductionin defects, there can be obtained an effect that the formation oftransistors in the crystallized region is subject to less effects ofdefects no matter how the channel of the transistors is oriented, thuseliminating constraints on the orientation of the transistor placement.

Next, the semiconductor thin film crystallization device according tothis embodiment is explained. As shown in FIG. 1, a first laser beamemitted from the first laser oscillator 1 is changed by the firstintermediate optical system 2 so as to form a first beam spot 11 asshown in FIG. 2 on the substrate 6 (above-noted semiconductor thinfilm). It is noted here that the first intermediate optical system 2 maybe either an optical system that changes the beam into a plurality oflaser beams or an optical system that changes the beam into a singlelaser beam. As shown below, however, the change into a single laser beamis preferred.

As the first laser device, it is preferable to use a solid state laserdevice of pulsed oscillation, in particular, which includes YAG lasers(which normally mean Nd:YAG lasers), Nd.YVO₄ lasers, Nd:YAlO₃ lasers,ruby lasers, Ti:sapphire lasers, glass lasers and the like. With respectto its second harmonics, it is preferable to change into a single laserbeam by using the first intermediate optical system 2 when the pulsewidth is set to 10 ns to 200 ns and the repetition frequency ofoscillation is set to 1 kHz to 100 kHz.

This is because the substrate 6 can be irradiated from its one end tothe other end by using a beam spot of the single laser beam. As a resultof this, various effects described above can be produced.

More specifically, the moving speed of the substrate 6 is determined by“moving distance for one time irradiation” multiplied by “repetitionfrequency of oscillation of first, second laser oscillators 1, 3.”Accordingly, typically, the moving speed is 10 microns×10 kHz=100 mm/s,meaning that the whole substrate 6 can be crystallized at a practicalhigh speed. For example, even in the case of a several hundredmillimeter square glass substrate, only several seconds is required toirradiate the substrate 6 from its one end to the other end. Also, whilemoving the substrate 6 or the beam spots 11, 12, the crystallization bysuch a crystallization method as described above makes it possible toobtain a plurality of crystals 18 extending in one way and free fromridges on the substrate 6 as shown in FIG. 6. In this case, there are noridges in the region where the substrate 6 (beam spots 11, 12) is moved,so that flat crystals 18 can be obtained. Thus, in the case where thesubstrate 6 (beam spots 11, 12) is moved from one end to the other endof the substrate 6, it is possible to obtain a thoroughly ridge-free,flat crystallized region from one end to the other end of the substrate6. That is, the entire substrate 6 can be formed substantially intounidirectionally grown crystals and a ridge-free surface.

In the case of using a plurality of laser beams, as described above, itis impossible to obtain an irradiation method that allows the substrate6 to be irradiated from one end to the other end, and it is alsoimpossible to prevent occurrence of ridges.

A specific structure of the first intermediate optical system 2 isexplained below. FIGS. 8A and 8B show a structure of a first example ofthe first intermediate optical system 2. It is noted that FIG. 8A is afront view and FIG. 8B is a side view.

Convex lenses 31, 32 have a beam expander function, and are capable ofchanging the diameter of the first laser beam derived from the firstlaser oscillator 1. Then, the first laser beam changed in beam diameteris inputted to a holographic element 33 and converged onto the substrate6 by the holographic element 33 to form a first beam spot 11 of theshape illustrated in FIG. 2. In this case, the beam intensitydistribution at the first beam spot 11 shows shapes as indicated byreference numeral “34” in the drawings.

It is noted that, the intensity distribution within the first beam spot11 preferably rises (or falls) as abruptly as possible at the edgeportions of the first beam spot 11 in the shorter-side (widthwise)direction thereof, as described before. In the longer-side (lengthwise)direction of the first beam spot 11, preferably, the intensitydistribution is constant, and rises (or falls) at the edge portionsequivalently in the case of the shorter-side direction.

The intensity distribution of the first laser beam, although varyingdepending on the type of the laser device, shows in many cases Gaussiandistribution or a distribution similar to Gaussian distribution.Therefore, this Gaussian distribution needs to be changed into such anintensity distribution described above. With the constitution of thefirst intermediate optical system 2 shown in FIGS. 8A and 8B, thehologram 33 provides the changing function of intensity distribution.Specifically, periodic or aperiodic dips and bumps formed on the surfaceof the transparent holographic element 33 make it possible to diffract alaser beam toward a desired direction. The resulting diffracted light iscombined together to obtain a desired intensity distribution on thesubstrate 6.

FIGS. 9A and 9B show a structure of a second example of the firstintermediate optical system 2. FIG. 9A is a front view and FIG. 9B is aside view thereof.

Convex lenses 41, 42 have a beam expander function, and are capable ofchanging the diameter of the first laser beam derived from the firstlaser oscillator 1. Then, the first laser beam changed in beam diameteris inputted to a holographic element 43 and converged onto the substrate6 by the holographic element 43 and a cylindrical lens 44 to form afirst beam spot 11 of the shape illustrated in FIG. 2. In this case, thebeam intensity distribution at the first beam spot 11 shows shapes asindicated by reference numeral “45” in the drawings.

It is noted that the intensity distribution within the first beam spot11, as in the case of FIGS. 8A and 8B, preferably rises (or falls) asabruptly as possible at the edge portions of the first beam spot 11 inthe shorter-side (widthwise) direction thereof. In the longer-side(lengthwise) direction of the first beam spot 11, preferably, theintensity distribution is constant, and rises (or falls) at the edgeportions equivalently in the case of the shorter-side direction.

In the constitution of the first example of the first intermediateoptical system 2 shown in FIGS. 8A and 8B, the hologram 33 changes theintensity distribution 34 into a desired one. In contrast to this, inthe constitution of the second example shown in FIGS. 9A and 9B, achange into a desired intensity distribution 45 is fulfilled by amultifunction of the holographic element 43 and the cylindrical lens 44.It is noted that the cylindrical lens 44 is so placed that the directionof its generatrix coincides with the lengthwise direction of the firstbeam spot 11.

More specifically, as shown in FIG. 9B, the hologram 43 has a functionof uniformizing the intensity in the longer-side (lengthwise) directionof the first beam spot 11. In contrast to this, the cylindrical lens 44is placed so as to have no image-forming effect in this direction.Conversely, as shown in FIG. 9A, the cylindrical lens 44 has theimage-forming function in the shorter-side (widthwise) direction of thefirst beam spot 11, and converges the laser beam into a desired width.Thus, a desired intensity distribution is obtained on the substrate 6.

According to the constitution of this second example, the hologram 43needs only to have the function of intensity uniformization, and doesnot need to have the function of converging the first beam spot 11 inthe widthwise direction. In this case, the hologram 43 does not need tohave any optically large power, so that a relatively large pitch ofconcaves and convexes of the hologram can be taken. Accordingly, thehologram becomes easier to make up.

FIGS. 10A and 10B show the structure of a third example of the firstintermediate optical system 2. FIG. 10A is a front view and FIG. 10B isa side view.

Convex lenses 51, 52, having a beam expander function, are capable ofchanging the diameter of the first laser beam derived from the firstlaser oscillator 1. Then, the first laser beam changed in beam diameteris inputted to a holographic element 53, making a cylindrical lens 54irradiated with the holographic element 53.

The holographic element 53 has a function of intensity uniformization inthe longer-side (lengthwise) direction of the first beam spot 11 bymaking the laser beam diffracted toward a desired direction with use ofconcaves and convexes formed on its surface. In contrast to this, thecylindrical lens 54, as shown in FIGS. 10A to 10B, is placed in such anorientation as to have the image-forming function in the shorter-side(widthwise) direction of the first beam spot 11 and not to have theconvergence effect in the longer-side (lengthwise) direction. Then, thecylindrical lens 54 converges the laser beam, with which a slit 55 isirradiated once.

The slit 55, having a shape similar to that of the foregoing first beamspot 11 of a desired shape, gives a limitation on the intensitydistribution of the laser beam. Further, properly designing the slitconfiguration of the slit 55 makes it possible to form an intensitydistribution of any arbitrary configuration. The image of the slit 55 isfocused on the substrate 6 by the image-forming lens (group) 56. Thus, adesired intensity distribution 57 is obtained on the substrate 6.

According to this third example, an image of the slit 55 is focused bythe image-forming lens (group) 56. Therefore, the intensity distributionbecomes sharper at edge portions of the first beam spot 11, facilitatingthe obtainment of clear images. Further, selecting a slit configurationin the slit 55 makes it possible to obtain a beam spot of any arbitraryconfiguration, facilitating the optimization of crystallization.

Next, a specific structure of the second intermediate optical system 4that converges a laser beam derived from the second laser oscillator 3to form a second beam spot 12 on the substrate 6 is explained. FIG. 11shows a structure of a first example of the second intermediate opticalsystem 4.

Lenses 61, 62 have a beam expander function, and are capable of changingthe diameter of the second laser beam derived from the second laseroscillator 3. Then, the second laser beam changed in beam diameter isinputted to an intensity uniformizing element 63, and converged onto thesubstrate 6 by the intensity uniformizing element 63 to form the secondbeam spot 12. The intensity uniformizing element 63, which is composedof two aspherical lenses 63 a, 63 b, changes a laser beam having anintensity distribution derived from the second laser oscillator 3 into alaser beam of uniform intensity.

Normally, when a single-mode carbonic acid gas laser is used as thesecond laser device of the second laser oscillator 3, the resultingintensity distribution is Gaussian distribution. Therefore, ifirradiation is applied as it is, the action of crystallization variesdepending on the irradiation region. For this reason, uniformizing theintensity by the intensity uniformizing element 63 makes it possible toobtain a uniform crystallization action within the second beam spot 12.In this case, the second beam spot on the substrate 6 results in acircular beam spot 64 as shown in FIG. 12, the first beam spot 11 beingcontained in the second beam spot 64.

FIGS. 13A to 13C shows an action of the uniformizing element 63 in FIG.11 on light beams. Referring to FIG. 13B, light beam groups 65, 66 areinputted to an aspherical lens 63 a of the intensity uniformizingelement 63, being transmitted toward an aspherical lens 63 b. In thiscase, the intensity distribution of the inputted second laser beam formsgenerally Gaussian distribution as shown in FIG. 13A, where theintensity is higher in central portions and lower in peripheralportions. Then, after the transmission through the uniformizing element63, the intensity distribution is changed into a laser beam havinggenerally uniform intensity from central to peripheral portions as shownin FIG. 13C.

In other words, the light beam group 65, which is derived from a regionexhibiting a low intensity in Gaussian distribution, is inputted to aperipheral portion of the intensity uniformizing element 63 and directedtoward a roughly converging direction, as shown in FIG. 13B. Meanwhile,the light beam group 66, which is derived from a region exhibiting ahigh intensity, is inputted to a central portion of the intensityuniformizing element 63 and directed toward a roughly divergingdirection. Thus, the light beam group 65 derived from a low-intensityregion is converged so as to be increased in intensity, while the lightbeam group 66 derived from a high-intensity region is diverged so as tobe decreased in intensity, with a result that a uniform intensitydistribution can be obtained.

For specific fulfillment of this, in the intensity uniformizing element63, the first aspherical lens 63 a has such a cross-sectional shape thatis concave in central portion and convex in peripheral portion as shownin FIG. 13B, which makes it possible to obtain the aforementioneduniformization effect. However, this arrangement does not allow a laserbeam passed through the first aspherical lens 63 a to become a parallelbeam. Thus, the second aspherical lens 63 b is placed so as to get theparallel beam. It should be noted that the second aspherical lens 63 bmay be omitted if a generally parallel laser beam is obtained by theaspherical lens 63 a.

FIGS. 14A to 14C show an action of the intensity uniformizing element onlight beams in the second example of the second intermediate opticalsystem 4. In this second example, a holographic element 71 is used asthe intensity uniformizing element. Referring to FIG. 14B, light beamgroups 72, 73 are applied to the holographic element 71, and a laserbeam exhibiting an intensity distribution of Gaussian distribution asshown in FIG. 14A into a laser beam exhibiting a uniform intensitydistribution as shown in FIG. 14C. The holographic element 71 in thiscase, similar in function to the aspherical lenses 63 a, 63 b shown inFIGS. 13A to 13c, has a converging function for a peripheral-part lightbeam group and a diverging function for the central-part light beamgroup 73. These functions can be achieved by properly designing thepitch or depth of concaves and convexes provided on the hologramsurface.

When the holographic element 71 is used as in the second example, itbecomes possible to control not only the intensity distribution of thelaser beams but also the shape of the beam spots, unlike the case wherethe first aspherical lenses 63 a, 63 b are used as in the first example.As to the reason of this, since the first aspherical lenses 63 a, 63 bare rotationally symmetrical in cross-sectional shape, the beam spot tobe formed results in a circular shape 64 as shown in FIG. 12. Incontrast to this, in the case where the holographic element 71 is used,there are no constraints on the shape of the beam spot by virtue of thehologram pattern's arbitrariness.

FIG. 15 shows a beam spot shape on the substrate 6 with the use of theholographic element 71. Since there are no constraints on the shape ofthe second beam spot, both the first beam spot 11 and the second beamspot 75 may be made rectangular. Like this, setting not acircular-shaped second beam spot 64 as shown in FIG. 12 but arectangular-shaped second beam spot 75 allows the irradiation efficiencyto be enhanced, more desirably.

The invention being thus described, it will be obvious that theinvention may be varied in many ways. Such variations are not beregarded as a departure from the spirit and scope of the invention, andall such modifications as would be obvious to one skilled in the art areintended to be included within the scope of the following claims.

1. A semiconductor thin film crystallization device comprising: a firstlaser irradiation unit for emitting a first laser beam and forming astriped first beam spot having a specified width and a specified lengthon a semiconductor thin film on a substrate; a second laser irradiationunit for emitting a second laser beam and forming a second beam spothaving a shape containing the first beam spot on the semiconductor thinfilm; and a relative moving unit for moving the substrate in a widthwisedirection of the first beam spot relatively with respect to the two beamspots, wherein the second laser irradiation unit emits the second laserbeam at least while the first laser beam is being emitted, and maintainsintensity of the second laser beam constant at a level lower than amaximum intensity of the first laser beam and during a period from anemission start time to an emission end time, and a light intensitydistribution by the second laser irradiation unit within the second beamspot is constant at least in a portion overlapping with the first beamspot.
 2. The semiconductor thin film crystallization device as claimedin claim 1, wherein the specified width in the striped first beam spotis not less than 5 micron and not more than 50 microns.
 3. Thesemiconductor thin film crystallization device as claimed in claim 1,wherein the second laser beam emitted by the second laser irradiationunit is a carbonic acid gas laser beam.
 4. The semiconductor thin filmcrystallization device as claimed in claim 1, wherein the second laserirradiation unit emits the second laser beam in a pulsed manner.
 5. Thesemiconductor thin film crystallization device as claimed in claim 1,wherein emission frequency of the first laser beam by the first laserirradiation unit is not less than 1 kHz and not more than 100 kHz. 6.The semiconductor thin film crystallization device as claimed in claim1, wherein the relative moving unit is a substrate driver section onwhich the substrate is placed and which is enabled to move the substratefrom one end portion to other end portion of the substrate in awidthwise direction of the beam spot and to rotate the substrate by 90°.